Phenol resins are used for an adhesive, a molding material, a coating material, a photoresist material, a color developing material, an epoxy resin raw material, a curing agent for epoxy resins, etc., and in view of the excellent heat resistance and moisture resistance of resultant cured products, phenol resins are widely used as a curable resin composition containing a phenol resin as a main agent or a curable resin composition containing an epoxidized resin and a phenol resin as a curing agent for an epoxy resin in the electric and electronic field such as a semiconductor encapsulant, an insulating material for a printed circuit board, etc.
Among these various applications, in the field of printed circuit boards, the tendency toward higher densities due to narrowing of the wiring pitches of semiconductor devices becomes remarked with miniaturization and improvement in performance of electronic apparatuses. As a corresponding semiconductor mounting method, a flip-chip bonding system of bonding a semiconductor device and a substrate with solder balls is widely used. The flip-chip bonding system is a so-called reflow-system semiconductor mounting method in which solder balls are disposed between a circuit board and a semiconductor and the whole is heated to cause fused-junction between the circuit board and the semiconductor. Therefore, the circuit board is exposed to a high-heat environment during solder reflowing, and thus faulty connection may occur in wiring due to a decrease in elastic modulus of the circuit board at a high temperature. Therefore, a high-heat-resistant material capable of maintaining a high elastic modulus even at a high temperature is required as an insulating material used for printed circuit boards.
On the other hand, the insulating material for printed circuit boards has been mixed with a halogenated flame retardant such as bromine or the like, which is added for imparting flame retardancy together with an antimony compound. However, in recent efforts to environment and safety, there has been strong demand for an environmental safety-responsive flameproofing method without using the halogenated flame retardant concerned about dioxin generation and the antimony compound suspected for carcinogenicity.
In this way, an insulating material for printed circuit boards is required to have high degrees of heat resistance and flame retardancy, and naphthol resins produced by reaction between naphthol and formaldehyde are know as resin materials complying with the requirement (refer to Patent Literature 1 below).
When a naphthol resin described in Patent Literature 1 is used as a curing agent for an epoxy resin, a heat resistance-improving effect is observed in a cured product because of the stiffness of its chemical skeleton as compared with general phenol novolac resins, but flame retardancy is unsatisfactory. In particular, in use as a varnish for a laminated plate, interfacial peeling easily occurs in the laminated plate due to the low adhesion to a substrate such as a glass cloth.
In addition, as a technique using a naphthol resin as a curing agent for an epoxy resin, for example, Patent Literature 2 below discloses a technique for decreasing the melt viscosity of a semiconductor encapsulating material by using naphthol novolac containing excessive amounts of di- and tri-nuclear compounds, thereby improving workability. However, the naphthol resin described in Patent Literature 2 is easily decomposed under a high-heat condition and thus does not exhibit satisfactory heat resistance and flame retardancy.